Method for shortening the statistical measurement times in the domain of radioactivity measurements

ABSTRACT

Method for the detection of an exceeding of a predetermined limiting value in a radioactivity measurement. A total duration of measurement for a contamination measuring device is calculated. Several single measurements are performed with the measuring device, with duration of measurement shorter than the total duration of measurement. After each single measurement, a probability is calculated on the basis of the previously measured measurement values. If the calculated probability is smaller than or equal to the limiting value, a signal is generated and the procedure is ended. Otherwise, further single measurements are performed.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not Applicable.

BACKGROUND OF THE INVENTION

The invention is related to a method for the detection of an exceedingof a limiting value in a radioactivity measurement.

In the measurement of radioactive contaminations, the measurement valuesare compared with a limiting value which results from regulatory andlegal prescriptions, respectively. Due to the nature of radioactiveradiation, the measurement values are subject to periodical variations.Therefore statistical certainties are used for the measurement of acontamination, which are related to the so-called background measurementand the event measurement. In doing this, it is known to establish theduration of measurement in dependence of the necessary statisticalcertainty, the resulting limiting value and the value of the backgroundmeasurement. The duration of measurement for the fulfilment of thestatistical certainty is thus predefined already before the beginning ofthe actual measurement, and is calculated from the aforementionedparameters. The measurement value for the predetermined duration ofmeasurement is determined and compared with the limiting value for themeasurement value. If the measurement value is above the limiting value,an exceeding of the limiting value is stated. With the known method, itis disadvantageous that comparatively long durations of measurementresult, even when there is no contamination (net measurement rate).

From DE 42 40 535, a method is known for fast detection of a radiationsource with a gamma line, moving in relation to the receiver.

From U.S. Pat. No. 3,670,164, a plutonium detector for persons is known.With the plutonium detector, the background radiation is continuouslymeasured by a number of gamma detectors. The measured value is used asthe actual value for the background radiation upon the entrance of aperson into the detection zone, and accounted for in the measurement.

The invention has the objective to provide a method for the detection ofan exceeding of a limiting value, which promptly and reliably indicatesthe occurrence of an exceeding of a limiting value.

BRIEF SUMMARY OF THE INVENTION

In the inventive method, the exceeding of a limiting value in aradioactivity measurement is detected. In a first step, a maximumduration of measurement is calculated for a contamination measurementdevice used. Preferably, the duration of measurement for thecontamination measuring device is calculated with the aid of thedetection limit defined in DIN 25482/1. According to the invention,thereafter several single measurements are performed with the measuringdevice, each with a shorter duration of measurement. After each singlemeasurement, a probability is calculated on the basis of the previouslymeasured measurement values, that the mean value of all the measurementvalues exceeds the limiting value in the still remaining duration ofmeasurement. This probability can be calculated without big problems,using the pertinent mathematics of probability calculation. In the casethat the calculated probability is smaller or equal to a predeterminedcertainty, a signal is generated which stops the measurement. Thepredetermined certainty is formed by a numerical value and serves as acomparative value for the calculated probability. Thus, in this case, astatement with a sufficient reliability has been achieved by one orseveral single measurement(s) with a shorter duration of measurementthan the prescribed total duration of measurement. If the probabilitythat in the still remaining single measurements the mean value of allthe measuring values exceeds the limiting value is greater than thepredetermined certainty, a single measurement is performed anew and theprobability is determined anew taking into account the new measurementvalue. This procedure is repeated as long as either a signal isgenerated that no certainty exists, or until the sum of the durations ofthe single measurements reaches or exceeds the total duration ofmeasurement. In the latter case, the probability for an exceeding of thelimiting value is greater than the predetermined certainty, so that,dependent on the demanded statistical certainty for the measurement, anexceeding of the limiting value is then stated or excluded. Theinventive method is advantageous in that the duration of measurement canbe significantly shortened in many measurements of objects or persons,when the radiation is far below the limiting value. This has as aconsequence that e.g. sluices in nuclear power stations, in whichmeasurements are performed, work considerably faster and that morepersons and goods can be guided through them during one shift. It isimportant to note that the inventive method can make the statement thatan exceeding of a limiting value in a radioactivity measurement hasoccurred or not occurred, respectively, with the same statisticalcertainty as conventional methods. In the inventive method, the durationof measurement is shortened dependent on the previously performed singlemeasurements, when it can be excluded on the basis of the singlemeasurement(s), with the prescribed statistical certainty, that anexceeding of the limiting value still might occur. The inventive methodis designated as P²-method or as “Probability Propagation” by theapplicant. The inventive method provides a saving of the duration ofmeasurement, when after first single measurements the statement can bemade statistically with sufficient certainty that an exceeding of alimiting value is improbable.

A gaussian distribution is used for the distribution of the measurementvalues, particularly with measurement devices for β- and/or γ-radiation.At comparably low gross counting rates, as occurring e.g. on themeasurement of α-radiation, the Poisson- or binominal distribution isused for the measurement values.

The inventive method is explained in more detail by means of the singleFIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

While this invention may be embodied in many different forms, there aredescried in detail herein a specific preferred embodiment of theinvention. This description is an exemplification of the principles ofthe invention and is not intended to limit the invention to theparticular embodiment illustrated.

As already set forth, the duration of measurement for a contaminationmeasurement device is calculated in Germany from the detection limitdefined in DIN 254821/1. The detection limit is defined as$\rho_{n} = {{\left( {\kappa_{\alpha} + \kappa_{\beta}} \right) \cdot \sqrt{\rho_{0}\left( {\frac{1}{t_{0}} + \frac{1}{t_{b}}} \right)}} + {\left( {\kappa_{\alpha} + \kappa_{\beta}} \right)^{2} \cdot \left\lbrack {\frac{1}{t_{0}} + \frac{1}{t_{b}}} \right\rbrack}}$where

-   κ_(α), κ_(β): quantiles of the standard normal distribution-   ρ₀: expected value of the background counting rate-   ρ_(n): detection limit for the expected value of the net counting    rate-   t_(o): duration of measurement of the background measurement-   t_(b): duration of measurement of the gross effect measurement

Resolution of the equation for the detection limit yields the grossduration of measurement for the measurement device. In doing this, thedetector efficiency is accounted for in known manner. Thus, with givendetection limit the duration of measurement is established. The durationof measurement is subdivided into N measurement cycles, each measurementcycle being performed for the duration T.

In the inventive method, after each measurement the probability iscalculated which indicates whether on taking into account of thehitherto obtained measurement values, an exceeding of the limiting valuewill still be obtained in the remaining duration of measurement.

This can be clarified by a simple example: suppose ten singlemeasurements are performed and the limiting value to be checked betwelve. According to the method of the invention, at the end of eachsingle measurement it is calculated how big is the probability that inthe remaining single measurements an exceeding of the limiting valuewill still be obtained.

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Thus, after the first single measurement it is asked, for instance,whether the probability is greater than 0.95, that an exceeding of thelimiting value will still be obtained in the mean value, when the firstmeasurement value is 2. After the second single measurement, it is askedwhether the probability is greater than 0.95, that an exceeding of thelimiting value will still be obtained in the mean value, when the firstmeasurement value is 2 and the second measurement value is 3. Thisprocedure is continued, until the estimated probability for the failureof an exceeding of the limiting value is high enough with sufficientcertainty on the basis of the developed measurement values.

It will become still more conspicuous that this method provides a savingof time, when the background value is measured. If nine times zero ismeasured, it is evident without any difficulty that the tenthmeasurement value will very unlikely be greater than 120, and thusleading to an exceeding of the limiting value. It will demonstrably fallbelow a demanded probability of 0.95. For this, with known distributionof the measurement values from the first nine measurement values, theparameters of the distribution can be estimated for instance, and thenthe probability of a measuring value to be greater than 120 can bedetermined.

The devolution of the procedure is illustrated in the single FIGURE as aflow chart. In the procedure step 10 the magnitude T_(max) iscalculated. This magnitude may be, for instance, the before-specifiedduration of measurement t_(b). In a subsequent step 12, a measurementwith duration T is performed with T<T_(max). The probability p_(i) iscalculated for the measurement value and measurement values measuredalready previously, that the mean value {overscore (x)} of allmeasurements which are possible in the measurement time T_(max), issmaller than a limiting value x_(Gr), the basis of the calculation beingthe previous measurement values. For the determination of thisprobability, it is taken advantage that at big counting rates, a normaldistribution exists for the measurement values and at low counting ratesa Poisson- or binominal distribution exists.

The probability p_(i) determined in this manner is compared with anappointed statistical certainty of limit exceeding P_(Gr) in procedurestep 16. Thus, if the remaining probability p_(i) for limit exceeding issmaller than the appointed statistical certainty of limit exceedingP_(Gr), a signal S_(i) is generated in procedure step 18 which stops thetotal measurement. If this statement is not possible, the total durationof single measurement is subsequently compared with the maximum durationin procedure step 20. If it has been measured for a sufficiently longtime already, a signal S₂ is generated according to which theprobability p_(i) is greater than P_(Gr) and an exceeding of thelimiting value possibly exists. If the hitherto occurred duration ofmeasurement in procedure step 20 is not greater than the totalmeasurement duration, the procedure returns to procedure step 12 andrepeats the measurement for the duration of measurement T, which issmaller than T_(max).

It is obviously possible to arrange the duration of measurement of thesingle measuring procedures in a variable manner, and even to select itin dependence of previous measurement results.

The method was tested by simulations, in which an effectiveness increaseof about 30% at equal certainty could be achieved on the basis ofrealistically recorded measurement values. The method was tested withthe data of a person monitor of type RTM860TS of the applicant as apre-monitor of a German nuclear power station. In 36 days, approximately27 000 measurements were performed in doing this, which corresponds to13 500 accesses. The average duration of measurement according to DINwas 9.9 seconds per body side. This measurement time could be shortenedby 27.9% with the inventive method, which leads to a saving of 34minutes per day and device at 375 accesses per day on the average.

The above examples and disclosures are intended to be illustrative andnot exhaustive. These examples and description will suggest manyvariations and alternatives to one of ordinary skill in this art. Allthese alternatives and variations are intended to be included within thescope of the attached claims. Those familiar with the art may recognizeother equivalents to the specific embodiments described herein whichequivalents are also intended o be encompassed by the claims attachedhereto.

1. Method for the detection of an exceeding of a predetermined limitingvalue in a radioactivity measurement, characterized by the followingprocedure steps: a total duration of measurement for a contaminationmeasuring device is calculated, several single measurements areperformed with the measuring device, with duration of measurementshorter than the total duration of measurement. after each singlemeasurement, a probability is calculated on the basis of the previouslymeasured measurement values, that with one or several singlemeasurement(s) during a time period the mean value of all the singlemeasurements exceeds the limiting value, the time period having at leastthe length of the time still remaining to the attainment of the totalduration of measurement, in the case that the calculated probability issmaller than a predetermined certainty to exceed the limiting value inthe remaining duration of measurement, a signal is generated that anexceeding of the limiting value did not occur and the procedure isended, otherwise further single measurements are performed until eitherthe total duration of measurement is reached or exceeded or until in oneof the single measurements the calculated probability is smaller than orequal to the predetermined certainty, and the procedure is ended. 2.Method according to claim 1, characterized in that a normal distributionis used for the distribution of measurement values.
 3. Method accordingto claim 1, characterized in that the measuring device is constructedfor β-radiation and/or γ-radiation.
 4. Method according to claim 1,characterized in that with comparatively low counting rates, thePoisson- or binominal distribution is used, preferably when measuringα-radiation.
 5. Method according to one of claim 1, characterized inthat the total measurement duration is determined depending on thedetection limit, the detection limit being defined as$\rho_{n} = {{\left( {\kappa_{\alpha} + \kappa_{\beta}} \right) \cdot \sqrt{\rho_{0}\left( {\frac{1}{t_{0}} + \frac{1}{t_{b}}} \right)}} + {\left( {\kappa_{\alpha} + \kappa_{\beta}} \right)^{2} \cdot \left\lbrack {\frac{1}{t_{0}} + \frac{1}{t_{b}}} \right\rbrack}}$with κ_(α), κ_(β) quantiles of the standard normal distribution, ρ₀expected value of the background counting rate, ρ_(n) detection limitfor the expected value of the net counting rate, t₀ duration ofmeasurement of the background measurement and t_(b) duration ofmeasurement of the gross effect measurement.